Boot from USB w/ Grub2… computer is useless right now… help!!!

Question

I'm running Ubuntu 12.04 on my MacBook Air, and after a botched update attempt the OS won't boot & recovery mode won't boot. The only thing I can do is drop into the Grub2 prompt. I have a USB drive that I should be able to boot from, but after going through these commands:

set root=(hd1)
chainloader +1

I get an error that says "invalid efi path". My MacBook Air has no other OS installed, so I can't go into reFit or anything. What are my options here? I was able to get into BusyBox at one point this evening, but heck if I know how to get back to it lol. tl;dr: need to boot MacBook Air from USB drive; can only use grub2; chainloader +1 doesn't seem to work. Help?!

Answer 1

If you've got an OS X installation, you should be able to get it to boot by holding down the Option key (or Alt if you're using a PC keyboard) when the chime sounds when you first power on the computer. You may then be able to reconfigure GRUB, install rEFIt or rEFInd to help select your OS for recovery, or otherwise reconfigure the system. Another option might be to try a rEFIt or rEFInd boot CD; that might enable you to boot OS X.

The GRUB commands you specified (set root=(hd1) followed by chainloader +1) are suitable for some types of recovery on a BIOS-based system, but the invalid efi path error you got suggests that GRUB is installed in EFI mode, not in BIOS mode. It's not clear what you were hoping to accomplish with those commands (what OS you intended to boot), but the equivalent for EFI is likely to be to load another OS's boot loader and chainload to it, as in:

set root='(hd0,gpt2)'
chainloader /System/Library/CoreServices/boot.efi

This example might boot OS X, depending on where it's installed. Details will vary depending on your installation, of course.

If you need more help, I recommend you run the Boot Info Script from an emergency disc and post a link to the RESULTS.txt file that it produces. That will give us more information on your configuration, which is necessary for more detailed diagnostics.